I studied physics in college. My advisor who wrote a
relativity text, wasn't a big fan of cosmologists. They
tend to make claims far in excess of what there is evidence
for.

Personally it wouldn't surprise me one bit if dark matter
is simply overlooked normal dark matter (cold rocks). Or
is something not currently understood about gravity, or
even another as yet to be discovered force.

Of course it could really be invisible particles. But I've
not seen conclusive evidence to assume it must be.